1. Field of the Invention
This invention relates to a method of fabricating a secondary lithium ion battery. More particularly, it relates to a method of fabricating a secondary lithium ion battery which can have an arbitrary shape, a reduced thickness, and a reduced weight.
2. Description of the Related Art
In order to meet the demand for size and weight reduction of portable electronic equipment, it has been the most important subject to increase capacity of batteries used therein, and development and improvement of a variety of batteries have been proceeding. Out of available batteries, a secondary lithium ion battery is expected to achieve the highest capacity and will undergo successive improvements.
A secondary lithium ion battery primarily comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The secondary lithium ion batteries that have been put to practical use employ an ion conducting layer made of a porous film of polypropylene, etc. filled with an electrolytic solution.
In the currently available secondary lithium ion batteries, electrical connections among the positive electrode, the ion conducting layer, and the negative electrode are maintained by using a rigid case made of stainless steel, etc. to apply pressure to the electrode body. However, such a rigid case occupies a large proportion in the weight of a battery, limits the possibility of size and weight reduction, and makes it difficult to make a battery of arbitrary shape.
In order to achieve size-and weight reduction and freedom of shape design, it is necessary to contact the positive electrode and the negative electrode to the ion conducting layer and to retain the contact condition without applying pressure externally.
In this regard, U.S. Pat. No. 5,437,692 proposes forming an ion conducting layer of a lithium ion-conducting polymer and contacting positive and negative electrodes to the ion conducting layer via an adhesive containing a lithium compound. Further, WO95/15589 proposes forming a plastic ion conducting layer and contacting positive and negative electrodes via the plastic ion conducting layer.
However, the process of U.S. Pat. No. 5,437,692 neither provides sufficient adhesive strength nor achieves sufficient reduction in battery thickness, and the resulting battery have unsatisfactory battery characteristics, such as charge and discharge characteristics, due to high resistance to ionic conduction between the ion conducting layer and each of the positive and negative electrodes. Use of a plastic ion conducting layer for contacting the electrodes disclosed in WO95/15589 fails to obtain sufficient adhesive strength and does not lead to sufficient reduction in battery thickness.
The present invention has been completed in an attempt to solve the above problem. It provides a process for producing a secondary lithium ion battery, by which a positive electrode and a negative electrode can be intimately contacted to an ion conducting layer (separator) via an adhesive resin to secure sufficient adhesive strength between each electrode and the separator and to control the resistance to ionic conduction between each electrode and the separator within a level of that of batteries having a conventional rigid case.
The present invention relates to a method of mounting a laminate body of a secondary lithium ion battery, particularly in characterized in that a binder resin solution consisting mainly of polyvinylidene fluoride and a solvent is applied onto a separator instead of electrode.
A first aspect of the method of the present invention of fabricating a secondary lithium ion battery comprises the steps of:
forming a positive electrode and a negative electrode by providing a positive electrode active material layer and a negative electrode active material layer on respective current collectors, PA1 applying a binder resin solution consisting mainly of polyvinylidene fluoride and a solvent onto a separator, PA1 forming a battery laminate by contacting the positive electrode and the negative electrode each on each side of the separator and drying the laminate to evaporate the solvent, and PA1 impregnating the battery laminate with an electrolytic solution. PA1 covering the laminated body with a flexible package; PA1 fitting air-tightly outsides of the laminated body to the flexible package by exhausting the flexible package; PA1 injecting the lithium ion-containing electrolytic solution from an opening of the flexible package into said separator, said active material layers and said therebetween in the laminated body, and PA1 sealing the opening of the flexible package.
A second aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the first aspect wherein the binder resin solution consists of 3 to 25 parts by weight of polyvinylidene fluoride and diethylformamide as a solvent.
A third aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the first aspect wherein the binder resin solution consists of 5 to 15 parts by weight of polyvinylidene fluoride and diethylformamide as a solvent.
A fourth aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the second aspect wherein, the step of drying the battery laminate is carried out in an air stream at 80.degree. C. or lower.
A fifth aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the first aspect wherein the separator is subjected to a plasma treatment prior to the step of applying the binder resin solution.
A sixth aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the first aspect wherein the step of forming a battery laminate comprises a step of heating a laminated body while pressing.
A seventh aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the sixth aspect wherein the step of impregnating comprises a step of dipping the laminated body into the lithium ion-containing electrolytic solution and a step of injecting them while reducing pressure of the lithium ion-containing electrolytic solution.
An eighth aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the sixth aspect wherein the step of impregnating further comprises a step of drying the laminated body while heating.
A ninth aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the sixth aspect wherein the step of impregnating comprises steps of:
A tenth aspect of the method of the present invention of fabricating a secondary lithium ion battery is the method according to the ninth aspect wherein the flexible package is made of resin laminated aluminum and the step of sealing comprises a step of heat-pressing.